Synthetic hydrogels, cross-linked materials that typically consist of more than 50% water, are notoriously brittle and have poor mechanical properties, including low strain to break and toughness and high stress-strain hysteresis. (Tanaka, et al. 2005 Progress in Polymer Science 30, (1), 1-9; Calvert 2009 Advanced Materials 21, (7), 743-756.) In contrast, biological materials often have robust mechanical properties in the hydrated state, such as the rubber-like proteins that can be deformed to high degrees of strain without failing. (Gosline, et al. 2002 Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 357, (1418), 121-132; Munch, et al. 2008 Science 322, (5907), 1516-1520.) The proteins elastin and resilin are both water swollen (40-60%) yet have the remarkable ability to undergo significant reversible deformation with no energy loss, also known as having high resilience. Nature appears to exploit this property for mechanical energy storage that facilitates movement.
Resilin, which is even more resilient than elastin, hence its name, serves a variety of purposes, from being involved in the jumping mechanism in fleas to the flight system of dragonflies and the sound producing organs of the locust. (Gosline 1987 Rubber Chemistry and Technology 60, (3), 417-438.) First investigated by Weis-Fogh in the 1960s in the form of the dragonfly tendon, it was shown to be 92-97% resilient, which is greater than polybutadiene rubber (80%), perhaps the most prototypical synthetic elastomer. (Weis-Fogh 1960 Journal of Experimental Biology 37, (4), 889-907; Elvin, et al. 2005 Nature 437, (7061), 999-1002.) Studies on the structure of resilin have shown that the cross-linking chemistry is highly specific, occurring only through the tyrosine units, with approximately 40 to 60 amino acid residues (˜4 to 7 kDa) between junctions.8 In addition, resilin is an amorphous material, with no stable secondary structures within the crosslinked primary chains. (Elliott, et al. 1965 Journal of Molecular Biology 13, (3), 791-795; Weis-Fogh 1961 Journal of Molecular Biology 3, (5), 648-667.) It is this uniform network (narrowly defined molecular weight between cross-links as well as robust cross-linking chemistry), low cross-link density, and absence of secondary structure within the primary chains that are thought to be responsible for the remarkable elastomeric properties of resilin.
In recent years, recombinant protein methods have been used to prepare elastic protein materials similar to elastin and resilin with reasonable success. (Elvin, et al. 2005 Nature 437, (7061), 999-1002; Li, et al. 2011 Biomacromolecules 12, (6), 2302-2310.) Cross-linked recombinant rec1-resilin, with an equilibrium water content of approximately 80%, was found to have a modulus of 2.5 kPa and could be stretched to 300% of its original length with negligible hysteresis upon removal of the load (resilience of 97%). (Elvin, et al. 2005 Nature 437, (7061), 999-1002.) Resilin-like polypeptide (RLP)-based elastomers with biologically active domains were also synthesized through a recombinant modular approach by Charati and coworkers. (Charati, et al. 2009 Soft Matter 5, (18), 3412-3416.) When hydrated, these materials had a water content of 85% and a Young's modulus (in tension) of 30-60 kPa. Further work on these hydrogels demonstrated that their properties could be tuned while maintaining a high resilience (>90%), even when extended to 200% strain. (Li, et al. 2011 Biomacromolecules 12, (6), 2302-2310; Charati, et al. 2009 Soft Matter 5, (18), 3412-3416.)
Nevertheless, these materials are protein-based. At the same time, a number of approaches have been employed to improve the mechanical properties of non protein-based synthetic hydrogels, often in an effort to match the performance of natural tissues. (Guan, et al. 2004 Journal of the American Chemical Society 126, (7), 2058-2065; Chen, et al. 2010 Journal of the American Chemical Society 132, (13), 4577-4579.) However, there has been significantly less progress toward the creation of materials with resilience similar to resilin. One reason for this is that many conventional crosslinking methods create inhomogeneous network structures with defects such as loops and dangling chains. (Malkoch, et al. 2006 Chemical Communications (26), 2774-2776.)
Efforts to improve the mechanical performance of hydrogels have involved the manipulation of network microstructures, as seen in nanocomposite, block copolymer, and double network (DN) gels, and the use of amphiphilic systems, where the hydrophobic component influenced the water content and mechanical properties. (Tanaka, et al. 2005 Progress in Polymer Science 30, (1), 1-9; Xiao, et al. 2010 Soft Matter 6, (21), 5293-5297; Johnson, et al. 2010 Progress in Polymer Science 35, (3), 332-337; Zhu, et al. 2006 Macromolecular Rapid Communications 27, (13), 1023-1028; Webber, et al. 2007 Macromolecules 40, (8), 2919-2927; Hou, et al. 2010 Biomacromolecules 11, (3), 648-56; Guo, et al. 2010 Soft Matter 6, (19), 4807-4818; Mespouille, et al. 2009 Soft Matter 5, (24), 4878-4892; Erdodi, et al. 2006 Progress in Polymer Science 31, (1), 1-18.) Studies on DN gels composed of a rigid highly cross-linked poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) first network and a loosely cross-linked polyacrylamide (PAAm) second network have shown that these gels have a high tensile strength and toughness, but exhibit considerable hysteresis after just one loading cycle. (Webber, et al. 2007 Macromolecules 40, (8), 2919-2927.) Similar large hysteresis loops have been observed for other gels, such as triblock copolymer gels, hybrid hydrogels, and hydrophobically functionalized polyelectrolyte hydrogels. (Seitz, et al. 2009 Soft Matter 5, (2), 447-456; Lin, et al. 2010 Macromolecules 43, (5), 2554-2563; Miquelard-Gamier, et al. 2008 Soft Matter 4, (5), 1011-1023.)
Many current synthetic hydrogels with seemingly attractive mechanical properties are of limited practical use when resilience is required, as one instance of loading to high strain results in permanent deformation or fracture and irreversibly changes the material properties. Other efforts have included end-linking reactions, such as Michael-type additions and click chemistry, to synthesize well-defined network structures. (Malkoch, et al. 2006 Chemical Communications (26), 2774-2776; Lutolf, et al. 2003 Biomacromolecules 4, (3), 713-722; Gupta, et al. 2010 Nat Chem 2 (2), 138-45.) However, the mechanical performance of such networks has not been systematically investigated. In 2008, Sakai and coworkers reported a homogeneous gel formed by cross-linking symmetrical tetrahedron-like poly-(ethylene glycol) (PEG) macromonomers that were designed to minimize entanglements. (Sakai, et al. 2008 Macromolecules 41, (14), 5379-5384.) This design strategy was an effective way to improve the elastic properties of the materials; however, to date, this chemistry has been limited to PEG. (Sakai, et al. 2010 Macromol Rapid Commun 31, (22), 1954-9; Fukasawa, et al. 2010 Macromolecules 43, (9), 4370-4378.)
Thus, an ongoing need exists for highly resilient synthetic hydrogels with tunable properties.